European Patent Application No. EP 903891 discloses a wireless communication network. The network allows devices to exchange messages (called frames) by wireless communication.
The network is organized as a group of cells. Each cell contains one device called an access point. Other devices, called stations, dynamically associate themselves with the cells. Each station selects one of the access points with which the station is able to communicate and associates itself with the selected access point. Subsequently the stations in a cell communicate with the access point of the cell with which they are associated, but not directly with each other or with the access points of other cells.
The network uses various measures to allow a plurality of cells to operate in parallel and to allow a plurality of stations to operate in a cell. First of all, each cell operates with frames carried by signals with frequencies in its own frequency channel. Spatially adjacent or overlapping cells generally use different frequency channels to minimize interference between the cells, but since only a limited number of frequency channels is available some cells will inevitably use the same frequency channels.
Secondly, the devices monitor the signal strength of incoming signals. The devices process frames from the received signals only when the strength of the signals is above a first threshold (the Carrier Detect Threshold or CDT). The CDT discriminates against signals from devices in other cells than the cell to which the receiving device belongs. Thus, the devices in a cell are not unnecessarily occupied by frames from other cells, leaving the devices ready to receive frames from their own cell.
Thirdly each device monitors the strength of received wireless signals before it transmits a frame itself. The device defers transmission of the frame as long as the strength of the received signals is above a second threshold (the Defer Threshold or DT). The DT is designed to ensure that the device does not start transmission while another device in the cell is busy transmitting a frame. In the past the DT was preferably selected to be somewhat lower than the CDT, to ensure that a device will not start transmission during transmission of a frame that is recognized to be above the CDT by other devices in the cell.
Selection of the CDT effectively determines cell size. The devices in a cell are deaf to frames from other cells that are so far away that their signals are attenuated to a level below the CDT before these signals reach devices in the cell. EP 903891 discloses some criteria for setting the DT and CDT in terms of curves that relate the distance between stations to signal strength. This publication does not disclose by which means these thresholds are set, but in practice the thresholds are set manually when the access points of the network are installed. The CDT is set according to the distance to adjacent cells in the network and the DT is set relative to the CDT. Such a manual selection of the thresholds is inconvenient and moreover prone to be sub-optimal, particularly when existing thresholds are not updated after changes to the network such as the removal or addition of stations.